1. Field of the Invention
This invention relates to an exposure system, and more particularly to an exposure system which exposes a color photosensitive body to two-dimensional imagewise light by the use of an exposure head comprising a surface emitting element array.
2. Description of the Related Art
Conventionally, there has been known an exposure head which exposes a photosensitive body to light by the use of a surface emitting element array comprising a plurality of linear emitting element arrays arranged in a sub-scanning direction substantially perpendicular to a main scanning direction in which a plurality of light emitting elements are arranged at predetermined pitches in each linear emitting element array. Recently, there have been proposed various exposure heads in which organic EL (electroluminescent) elements are employed as the light emitting elements.
Further, there has been known, as disclosed in U.S. Pat. No. 6,731,322, an exposure system where the exposure head comprising the surface emitting element array formed by N linear emitting element arrays and the photosensitive body are moved relatively to each other in the sub-scanning direction to expose N times the same parts of the photosensitive body by the N linear emitting element arrays, thereby recording a two-dimensional image on the photosensitive body. By multiple exposure of the same parts of the photosensitive body, the dynamic range of exposure can be kept higher and the image can be recorded in a higher gradation. Further, in U.S. Pat. No. 6,731,322, there is disclosed to expose a color photosensitive body to a color image by the use of linear emitting element arrays emitting red, green and blue light in such an exposure system.
In the exposure system in which an exposure head comprising a surface emitting element array is employed, it is preferred that the light emitting characteristics of the plurality of light emitting elements be uniform in recording a fine image. Especially, if the amounts of light emission from the plurality of light emitting elements arranged in the main scanning direction are non-uniform, unevenness of density occurs in the printed image since the pixels are exposed to non-uniform light from the light emitting elements corresponding thereto even if the light emitting elements are driven on the basis of a given image signal. This unevenness of density is generally called ‘score unevenness’ since it extends like a score in the sub-scanning direction as the sub-scanning progresses.
In the exposure system described above where the same parts of the photosensitive body are exposed a multiple of times, N times, non-uniformity in the amounts of light emission in the main scanning direction is uniformed by superposing light from a plurality of light emitting elements and accordingly, occurrence of the score unevenness is suppressed. However, in the conventional exposure systems, the score unevenness is apt to occur in the part of cyan or magenta even if the arrangement of the multiple exposure is applied and in order to excellently suppress the score unevenness, it is necessary to arrange a great number of linear emitting element arrays in the sub-scanning direction.